Scroll compressors have emerged as a popular alternative to reciprocating compressors and are generally available in hermetic configurations in capacities up to 30 tons. Multiple scroll compressors are often used in a single chiller to meet larger capacities and can offer as much as 30 stages of capacity. In general, scroll compressors are more efficient than reciprocating compressors and have proven to be very reliable, primarily because they have approximately 60 percent fewer moving parts than reciprocating compressors. Reciprocating and scroll compressors are typically used in smaller water chillers. Scroll compressors also offer stepped or discreet unloading capabilities, while screw and centrifugal compressors have proportional unloading. However, all compressor types are now starting to be offered with variable frequency drive.
Helical-rotary (screw) compressors
Helical-rotary (screw) compressors have been used for many years in air compression and low-temperature-refrigeration applications. They are now widely used in medium-sized water chillers. Helical-rotary compressors have a reliability advantage due to fewer moving parts and a wide operating map; they also operate for comfort cooling, medium temperature solutions, and high condensing temperatures. They have capacity control that use stepped unloading, proportional slide valves, and recently variable frequency electric drives.
Centrifugal compressors have long been used in larger tonnage water chillers. High efficiency, superior reliability, reduced sound levels, and relatively low-costs have contributed to the popularity of the centrifugal chiller. Centrifugal compressors are generally available in prefabricated chillers from 100 to 3,000 tons [350 to 10,500 kW], and up to 8,500 tons [30,000 kW] as built-up machines. The capacity of a centrifugal chiller can be modulated using inlet guide vanes (IGV) or a combination of IGV and a variable-speed drive. Other common names for variable-speed drive include adjustable-frequency drive (AFD), and variable frequency drive (VFD).
Variable Speed Drives
Variable-speed drives are widely used with fans and pumps, as well as centrifugal, scroll and screw chillers. As a result of the advancement of microprocessor-based controls for chillers, they are now being applied to water chillers. Using an AFD with a chiller can degrade the chiller’s full-load efficiency but can greatly enhance part load efficiency by reducing motor speed at low-load conditions, such as when cooler condenser water or cooler ambient for air cooled is available or a building is not fully occupied. For centrifugal compressors, it is important that the lift decrease with speed reduction to control capacity, which is why both inlet guidevanes and variable speed are required. For screw and scroll compressors, which are positive displacement compressors, speed alone can be used for capacity control.
Certain system characteristics favor the application of an adjustable-frequency drive, including:
A substantial number of part-load operating hours
The availability of cooler condenser water for centrifugals, but not for positive displacement
Chilled-water reset control
Low electrical utility demand charges or no peak load control systems
An evaporative chiller is a modified hybrid version of an air-cooled condenser and a cooling tower. In evaporative chillers, the refrigerant is running through tubes and air is drawn or blown over the tubes by a fan; however, in an evaporative chiller, the water is sprayed on the tube surfaces to take advantage of the heat of absorption of water. As the air and water pass over the coil, it causes the water to evaporate, which results in cooler air temperatures and higher efficiency of the condenser. The evaporation process and cooler air absorbs heat from the coil, causing the refrigerant vapor within the tubes to condense. The remaining water then falls to the sump to be recirculated and used again. This application is best suited in dry, hot climates, similar to water cooled chillers using cooling towers.
Absorption water chillers
Absorption water chillers differ from vapor-compression chillers in a couple of ways, the first being that they use heat energy as the primary driving force. The heat can be in the form of steam or hot water (indirect-fired), or by oil or natural gas (direct-fired). Secondly, absorption water chillers do not have a compressor; the compressor is replaced by an absorber, a pump, and a generator. In addition to the refrigerant, which in this case is water, the absorption water chiller uses a secondary fluid called an absorbent. The condenser, expansion device, and evaporator sections, however, are similar. Absorption water chillers generally have a higher initial cost than vapor-compression chillers because of the additional heat-transfer tubes required in the absorber and generator(s), the solution heat exchangers, and the cost of the absorbent. This initial cost premium is often justified when there are limited electricity resources and/ or the cost of electricity is high. Because electric demand charges are often highest at the same time as peak cooling requirements, absorption chillers are often selected as peaking or demand-limiting chillers. Backup generator capacity requirements may be lower with absorption chillers than with electrically-driven chillers as the absorption chiller uses only a small amount of electricity. This makes absorption chillers attractive in applications requiring emergency cooling, assuming the alternate energy source is available.
Absorption chillers can also be used in facilities, such as hospitals or factories, where excess steam or hot water result as part of normal operations. Rather than wasting this energy, it is used as fuel, similar to a gas turbine, generate waste steam, or some other waste gas that can be burned. Cogeneration systems often use absorption chillers as a part of their total energy approach to supplying electricity in addition to comfort cooling and heating.
Heat Pump Chiller Packages
Chillers also can be used to produce heat. Water-to-water chillers are used to both cool and heat. The cooled water can be used to cool portions of the building that need cooling and dehumidification while the warm condenser water can be used to heat other areas within the building. They also can be used to cool a process load and provide heat to other requirements or processes, including domestic water. The cooled source can be external to the building and could include river or ground water. Air-to-water reversible heat pumps can be used to cool water during warm seasons, and then reversed to heat water during colder months. In multiple chiller operations they can be used as the first chiller when buildings often operate with cooling and heating loads, and can provide efficiency improvements over separate heating and cooling systems.
AHRI 550/590 standardizes the method that is utilized to evaluate the performance of a water-chilling package using the vapor compression cycle. It enables customers to efficiently and accurately compare different packages. It standardizes the rating of energy efficiencies, water pressure drops, integrated part load values (IPLV), and non-standard part load values (NPLV), while also determining capacities. This standard is applicable to open drive/hermetic motors, which include centrifugal, rotary screw, reciprocating, and other types of compressors in various types of chiller packages. Download AHRI Standards here.
Need for code
The publication of ASHRAE 90.1-1988 includes an AHRI Full-Load and IPLV rating at standard rating conditions, and is the widely accepted metric used to compare relative chiller efficiencies. This code enables chiller customers to easily compare between chiller equipment since they all use the same rating system. The code enables customers to quickly reference both Full-load and the Part-load (IPLV) of chillers that meet their requirements.
In 2010, a Path B was added to the standard as an alternate compliance option for part load intensive water-cooled chillers. Also, several categories were eliminated, including the separate category for reciprocating water-cooled chillers and the category for condenser-less chillers. The scope was expanded to cover positive displacement chillers with a setpoint about 32°F that are charged with a fluid that protects against freezing.
In 2012, AHRI’s Liquid Chillers Section unanimously approved proposed changes to further improve efficiency that would be added to ASHRAE 90.1 -2013 with an effective date of January 1, 2015. Some of the proposed changes include: adding a Path B for air-cooled chillers; modifying some of the capacity ranges; and moving the efficiency requirements for water-cooled positive displacement and centrifugal chillers closer together.
This standard ensures the repeatability of energy efficiency in manufactured chilling products. Without this code, manufacturers would not have a reference for publishing claimed efficiencies, including the ability to test to published performance levels.
The origins of AHRI Standard, Centrifugal and Rotary Screw Water Chillers, can be traced back to 1963; likewise, AHRI Standard, Reciprocating Water Chillers, began in 1962.
AHRI’s Liquid Chillers Certification Program was launched in 1990 and tested units to AHRI Standard 550-1990, Centrifugal and Rotary Screw Water Chillers. Certification to AHRI Standard 590-1992, Positive Displacement Compressor Water Chillers, began in 1995. In response to work being done by the ASHRAE Standard 90.1 Committee, efforts soon began to update and combine the two ARI standards into one more comprehensive chillers standard, and develop a part load efficiency metric, now known as Integrated Part Load Value (IPLV). The combined standard was first published in 1998 and was quickly updated with addenda.
In 2003, the 1998 version was reaffirmed, the addenda were adopted and the standard was re-released. The next edits began and the standard received significant work, in large part due to the fact that chiller technology had improved and expanded since the last true revisions of the standard.
This standard outlines the method used to evaluate an absorption water chiller/water heating packages, allowing customers to properly and efficiently evaluate products. It standardizes the rating of water pressure drops, integrated part load values (IPLV), application part load values (APLV), and determines capacities. This benchmark applies to single/double-effect steam and hot fluid operated water chilling units, double-effect direct-fired (natural gas, oil, LP gas) water chilling/heating units as well as multiple-effect and multi-loop cycle absorption water chilling/heating units.
The purpose of this standard is to provide minimum U.S. requirements for the energy-efficient design of buildings except low-rise residential buildings. The standard is applied to new constructions, retrofitted older buildings, and new systems. It will be applied towards all major components of building management, such as: power distribution, environmental control, and resource management.
This standard helps engineers select equipment that has low power efficiency and better management systems in a world where bottom lines are becoming tighter by the day.
The purpose of this standard is to enable the design of energy-efficient building envelopes as well as other systems, such as the power, lighting, and mechanical systems. This standard emphasizes optimal performance and maximum efficiencies, and deters users from wasting finite resources. It is similar to ASHRAE Standard 90.1 and has the same requirements for chillers.